Critical Po-2 of skeletal muscle in vivo

The main purpose of this study was to determine the interstitial oxygen ten sion at which aerobic metabolism becomes limited (critical Po-2) in vivo in resting skeletal muscle. Using an intravital microscope system, we determi ned the interstitial oxygen tension at 20-mu m-diameter tissue sites in rat spinotrapezius muscle from the phosphorescence lifetime decay of a metallo porphyrin probe during a 1-min stoppage of muscle blood flow. In paired exp eriments NADH fluorescence was measured at the same sites during flow stopp age. NADH fluorescence rose significantly above control when interstitial P o-2 fell to 2.9 +/- 0.5 mmHg (n = 13) and was not significantly different ( 2.4 +/- 0.5 mmHg) when the two variables were first averaged for all sites and then compared. Similar values were obtained using the abrupt change in rate of Po-2 decline as the criterion for critical Po-2. With a similar pro tocol, we determined that NADH rose significantly at a tissue site centered 30 mu m from a collecting venule when intravascular Po-2 fell to 7.2 +/- 1 .5 mmHg. The values for critical interstitial and critical intravascular Po -2 are well below those reported during free blood flow in this and in othe r muscle preparations, suggesting that oxygen delivery is regulated at leve ls well above the minimum required for oxidative metabolism. The extracellu lar critical Po-2 found in this study is slightly greater than previously f ound in vitro, possibly due to differing local conditions rather than a dif ference in metabolic set point for the mitochondria.